Heavy oil represents a substantial fraction of world petroleum reserves. Conventional production practices cannot recover a large percentage of this oil partly because high viscosity at reservoir conditions inhibits flow to production wells. Therefore, ultimate recovery by conventional production is frequently below 10%. Steam injection is the most widely utilized method for stimulating the production of heavy oil, currently accounting for approximately 80%. A benefit of using steam as a heat transfer medium is the large quantity of heat released when it condenses into water. With a latent heat of vaporization (or condensation) as high as 1,000 BTU per pound, very little steam carries a large quantity of energy. Other advantages include the safe, nontoxic and nonflammable characteristics of steam, in addition to its ability to deliver heat at a constant, controlled temperature.
Steam-flooding involves injection of two-phase steam having sufficient quality at effective rates into a reservoir. However, the cost for generating steam is high, accounting for about one-half of all steam-flooding operating costs. Because of the high steam cost and the difficulty in obtaining generator permits, optimization of the use of injected steam is necessary. However, the quality or steam delivered to the reservoir is largely unknown, which makes steam quality measurements important for both oil extraction and reservoir management. Such determinations are also important in other industries where steam is used.
Water can exist as either a gas or a liquid under saturated conditions. Wet steam can contain both gas and liquid components, known to those of ordinary skill in the art as two-phase flow. A common method of expressing the quantities of each phase, known as steam quality, is the ratio of the mass flow rate of the gas phase to the total mass flow rate, and is given as a number less than one, or as a percentage. Steam quality measurements, which are determinative of the efficiency of the steam delivery system in surface distribution lines, have been made using various methods. One technique is based on pressure drop measurements as the steam passes through an area constriction, and requires the accurate measurement of pressures with pressure measurement devices within the pipeline. Another method bleeds steam to the outside of a pipe through an orifice, for generating acoustic energy, which is detected. The amplitude of the detected signal is related to steam quality. However, this method produces unnecessary noise and releases steam into the environment. Capacitance measurements for determining steam quality, where the measurement apparatus is inserted into a steam carrying pipe, also requires an additional temperature or pressure measurement device. Measurements of differential pressure fluctuations on both sides of a metal plate containing an orifice inserted into a steam carrying pipe to determine steam quality also require pressure measurement devices connected through holes in the pipe.
Optical on-line measurement systems have been used as well for determining steam quality. Multiple wavelengths of radiant energy are passed through the steam from an emitter to a detector through optical windows in a pipe. By comparing the amount of radiant energy absorbed by the flow of steam for each wavelength, an accurate measurement of the steam quality can be determined on a continuous basis in real-time.
Since these approaches require either penetration of the steam-carrying pipes or insertion of devices into the pipe, none provides a simple and noninvasive method for monitoring steam quality that can be installed at one location and then readily moved to another location without significant plumbing changes, often rendering these methods unsuitable for field use. Additionally, steam conduits are operated in excess of 400° F., and measurement devices must withstand high temperatures.
Approximate measurements of steam flow rate or steam quality parameters, are often sufficient for field steam injection operations, and the ability to make noninvasive measurements at multiple locations using inexpensive, easily maintained and easily automated devices is of value.